Thursday, February 12, 2015

Sexual Cannibalism: Putting 50 Shades of Grey to Shame

By Jackie Campbell

With Valentine’s Day right around the corner, what better way
to get into the spirit than reading about sexual cannibalism! Fifty shades of
Grey can’t compare with these animal extremes that’s for sure.

Sexual cannibalism (SC) typically in nature refers to
consumption of a male by his mate before, during or after copulation. SC is
usually associated with females as they are the child-rearing individuals. In
other words, it would not make sense evolutionarily for males to consume the
individuals that will be rearing their children after insemination. SC is a
common practice in many species of praying mantis; however, the rates of SC can
vary among species. For example, some species can have a rate as high
as 46% as in the Australian praying mantis Pseudomantis albofimbriata. Conversely, the genus Ciulfina lacks
sexual cannibalism altogether. The practice of SC is not exclusive to species
of praying mantis either. A wide array of species of spiders have been shown to
demonstrate SC as well, sometimes even in the reverse direction (males
cannibalizing females!). So what makes this practice of sexual cannibalism
evolutionarily advantageous?

At first thought, it seems that
SC would be disadvantageous as it would decrease the sex ratio of the species
and eliminate genetic variability. Aka eliminating any potential future dates
such as those needed for Valentine’s Day especially since the pickings are
already slim enough as it is! *gasp* Thus, the advantage of SC lies in the
nutritional benefits that can come from eating protein-rich and lipid-rich
conspecifics. In fact, females in many species of praying mantis use the
nutrients gained from cannibalism to increase body condition and reproductive
output. Female body condition can improve by as much as 33% while fecundity
(reproductive rate/success) can improve by as much as 40% after the consumption
of a male conspecific. Additionally, this cannibalism can also provide more
energy for offspring investment, increasing the survival of offspring. In a
study by Welke and Schneider 2012, the effects of sexual cannibalism on clutch
size and offspring survival success were observed in female orb-web spiders of
the species Argiope
bruennichi. The results of the study showed that female clutch
size, a measure of fecundity, was higher in females allowed to consume mates
compared to other females not allowed to consume mates (Figure 1). Eggs produced
by female orb web spiders also had higher yolk density providing more nutrients
to offspring and thus increasing their survival rate (Figure 1). After this
description many of you may be thinking that the males are really drawing the
short end of the stick in this situation. As with most males attempting to buy
the “perfect” valentine’s day gift, the males of these sexually cannibalistic
species appear to be in a lose-lose situation. However, males seem to know what
they are getting themselves into and tend to discriminate when it comes to mate
selection.

Males of sexually cannibalistic species tend to
discriminate when choosing potential female mates, especially when the risk of
being trapped and eaten is a high possibility. A study conducted by Brown et al
2012 looked into the different mating approaches of males in sexually
cannibalistic species. Males’ choice of mates was dependent upon two
factors: proportion of risky females and frequency of mate encounters. The
results of the study showed that males tended to discriminate more when the
overall cost of approaching females was high. Males were tested under both high
risk conditions (encountering starved females) and low risk conditions
(encountering fed females). Overall, in the low risk conditions, males tended
to discriminate less and approached females more frequently for copulation
(Figure 2). Conversely, males that were paired with unfed females tended to
approach the females slowly or avoided the females altogether. In addition to the risk of the situation,
another variable was also tested. The variable of frequency of mate encounters.

In a second experiment, risk and access to mates were compared
to survival of males (Figure 3). In this case, males that had high access to
mates exhibited the same pattern as in the previous experiment. However, males
with lower access to mates and were exposed to unfed/high risk females
expressed a different pattern. The low encounter males had decreased survival rate,
as they tended to approach females regardless of their risk level (Figure 3).

Figure 3. Mortality rate of male praying mantis compared to high and low risk females and accessibility to females.

It appears that
these males were risk takers and the costs of not having the opportunity to
mate during this breeding season out-weighed the risks of becoming
cannibalized. These male praying mantids had their priorities straight that’s
for sure.

In sexually cannibalistic
species, the strategy of copulation is as follows: females remain sedentary and
males go out and search for mates. In order to attract males, females release
chemical signals known as pheromones to attract males to them for copulation.
As was demonstrated above the males of these species of praying mantids have
become quite discriminatory in their mate choice. With this in mind, it is no
wonder that males of many species of praying mantids can detect differences in
body condition and fecundity of females. Thus, males tend to choose females
that have good body condition and high fecundity. These males want to make sure
they are getting the most bank for their buck especially if there is a chance
of having only one copulation event.

What most males fail to
understand is that with every “sneaky” strategy they think they are coming up
with, there is a female that is always one step ahead of them. We females tend
to be a wily bunch and know how to exploit sexually-motivated males as easy
prey. These male praying mantids think they are smart and can detect the
difference between poor condition females and good condition females but a
study conducted by Barry 2014 begs to differ. In this study, females of varying
body conditions were isolated with males and their success of attracting males
was determined. According to Figure 4, females that were in good condition
attracted the most amount of males but not as much as the very poor condition
females. How can this be? Well, as was stated earlier, these poor condition
females can improve the quality of their body condition by eating a
nutrient-rich conspecific.

Figure 4.
Representation of attractiveness of females (filled bars) and average body
conditions (unfilled bars) and the amount of males attracted.

Instead of producing eggs, these females
produce higher levels of pheromones compared to the good body conditioned
females. Despite the fact that these pheromones are costly energetically to
produce and their production in high amounts can affect both survival and
future reproduction, females that are of already poor body quality have nothing
to lose. The risks of potentially getting a nutrient-rich meal far out-weighs
the costs of pheromone production. Poor males, bamboozled yet again. But in
retrospect, do we really think these males care a whole ton? The plus side of
sexual cannibalism is these males can avoid long-term commitments and future Valentine’s
Day expectations. Win-win.

For more on the awesome praying mantis, check
out this extremely scientifically accurate and respected video!